This invention relates to a process and apparatus for analyzing a sample of a material of unknown composition. In particular, the invention relates to the preparation of a sample for analysis by irradiating it with energy emitted by a pulsed laser beam.
German Offenlegungsschrift No. 1,598,632 discloses a process wherein a sample to be analyzed is exposed to electromagnetic radiation, especially coherent radiation, thereby producing vapors, plasmas, ion beams, and the like. These products are then subjected to emission or absorption spectroscopic analysis and/or mass spectroscopy. In carrying out this process, the laser light is focused by means of a collecting lens onto that portion of the surface area of the sample which faces the analyzing means.
A prerequisite for such an arrangement is that the sample material be essentially transparent to the light emitted by the laser. If the sample is not transparent to laser light, it must be so thin that the laser burst will produce a hole in the sample. Then, the sample particles to be analyzed can pass through the sample to the side facing away from the collecting lens where the analyzing device is located.
One disadvantage of the prior art arrangement is that not every sample can be fabricated as a thin film. Such a sample can be viewed only in incident light while aiming the laser beam; thus, the side of the sample facing the analyzer is not visible. Consequently, data indicating which part of a sample, which frequently has a specific structure, has been excited by the laser beam and vaporized are fraught with uncertainties.
Arrangements are also known wherein nontransparent samples are irradiated with a laser beam in the presence of incident light, and the thus-produced ions transferred directly by suction into a mass analyzer. This method has the disadvantage that the collecting lens system focusing the laser light onto the sample, as well as the inlet opening of the mass analyzer (or of an ion optical system connected in front of the mass analyzer), must be on the same side of the sample. Since both devices occupy a large amount of space, the collecting lens systems and inlet opening are at a relatively great distance from the location on the sample at which the laser beam impinges. As a consequence, it is impossible to direct the laser beam to particularly small zones of the sample, although this is desirable in laser microscopy wherein the irradiated zone can be reduced as the sample is brought closer to the collecting lens system. Further, a relatively large spacing between the sample and the analyzing device results in poor acceptance of the emitted ions.